The present invention relates to a method for producing a rocker arm made of light alloy for use in an overhead cam shaft type valve operating mechanism of an internal combustion engine through a die casting process.
A rocker arm for use in an overhead cam shaft type valve operating mechanism of an internal combustion engine includes at its central portion a thick-walled cylindrical bearing section which fits on a rocker arm shaft, an abutting surface adapted to abut against a valve drive cam is formed at an end portion of one arm, and a threaded bore for mating with a valve clearance adjusting screw is formed at an end portion of the other arm. This rocker arm is normally formed of cast iron, malleable cast iron, nodular graphite cast iron, etc., and besides in recent years, it is provided also as a forged article. In order to give wear resistance to the abutting surface, in the case of a cast article the surface layer is hardened as by chill or flame hardening, and in the case of a forged article it is subjected to hardening treatment such as flame hardening, induction hardening, chromium-plating, etc. In addition, as another procedure for giving wear resistance to the abutting surface is to provide a wear-resistive piece made of sintered material, chill casting plate or the like.
On the other hand, with regard to structural member for use in an internal combustion engine, reduction in weight is being advanced positively by employment of light alloy materials such as aluminum-alloys, magnesium-alloys, etc. To form moving parts such as rocker arms of light alloy material is effective in that the inertia force is reduced. Especially it is advantageous to form rocker arms for use in an overhead cam shaft type valve operating mechanism which is preferable for a high-speed engine, of light alloy material.
FIG. 1 shows a prior art rocker arm 01 made of light alloy. This rocker arm 01 was produced through a die casting process, it includes at its central portion a thick-walled bearing section 02 in which a rocker arm shaft hole 03 adapted to fit around a rocker arm shaft is formed (it being formed by drilling a hole in a die cast article), at an end of an arm 04 is integrally joined a wear-resistive piece 05 (made of iron-series sintered material, ceramics, etc.) for abutting against a valve drive cam by cast-encircling, and at an end of an arm 06 is formed a threaded bore 07 for a variable clearance adjusting screw.
The above-mentioned bearing section 02 must be formed with a thick wall so that it can withstand large stress repeatedly applied thereto, and hence, upon casting there is a possibility that many blow holes may be produced within the bearing section.
In the case of making the rocker arm 01 shown in FIG. 1 through a die cast process, a sprue of a metal mold would be provided at a position indicated by arrow A or arrow B, and also the direction of the sprue is determined in the direction of the above-described arrow. By providing such a sprue, molten metal can be effectively fed to the bearing section 02 which requires a relatively large amount of molten metal, and to the end portion of the arm that is most remote from the sprue.
However, in the case where the sprue A is employed, since the wear-resistive piece 05 is positioned sideways with respect to the direction of feeding molten metal, feeding of molten metal to the periphery of the wear-resistive piece 05 having a protrusion is retarded, and so, a cast-encircling structure having an excellent adhesion property cannot be obtained. In the case where the sprue B is employed, since the wear-resistive piece 05 is positioned at the most remote locations from the sprue, feeding of molten metal to the periphery of the wear-resistive piece 05 is poor, and in this case also, an excellent cast-encircling structure cannot be obtained. Also, in either case, there is a possibility that displacement of the wear-resistive piece 05 may occur while pouring molten metal.